Power semiconductor devices are classified into Schottky barrier diodes, metal semiconductor field effect transistors, and high electron mobility transistors (HEMTs).
An HEMT is widely used due to excellent electron mobility and low noise characteristics thereof as an integrated circuit device operating at ultra high frequencies up to millimeter wave frequencies. As application systems employing HEMTs become more complex and elaborate, characteristics, particularly, radio frequency (RF) characteristics of HEMTs need to be improved.
Maximum oscillation frequency (Fmax) is a very important factor to evaluate RF characteristics of the HEMT. The maximum oscillation frequency (Fmax) may be improved by adjusting small-signal parameters and improving DC characteristics. There are a lot of other variables affecting the DC characteristics and small-signal parameters of the HEMT. Among these, a gate-recess structure, as the most important factor, will be described hereinafter.
FIG. 1 is a side view, in section, schematically illustrating a conventional HEMT device 1A having a first gate-recess structure with a wide recess region where a gate electrode is disposed. FIG. 2 is a side view, in section, schematically illustrating a conventional HEMT device 1B having a second gate-recess structure with a narrow recess region where a gate electrode is disposed.
Referring to FIGS. 1 and 2, each of the conventional HEMT devices 1A and 1B includes a substrate 10, a buffer layer 20 disposed on the substrate 10, a barrier layer 30 disposed on the buffer layer 20, and a cap layer 40 disposed on the barrier layer 3.
The conventional HEMT devices 1A and 1B respectively include recess regions R1 and R2 formed by partially removing the cap layer 40 to expose the barrier layer 30. A gate electrode 53 is disposed in each of the recess regions R1 and R2, and a source electrode 51 and a drain electrode 52 are disposed on the cap layer 40.
The HEMT device 1A illustrated in FIG. 1 has the first gate-recess structure with a wide recess region formed by partially removing the cap layer 40 except for regions on which the source electrode 51 and the drain electrode 52 are disposed. The HEMT device 1B illustrated in FIG. 2 has the second gate-recess structure with a narrow recess region formed by partially removing the cap layer 40 only at a region where the gate electrode 53 will be formed.
The HEMT device 1B having the second gate-recess structure has higher maximum drain current (Idss,max) and higher maximum transconductance (Gm,max) than the HEMT device 1A having the first gate-recess structure. This is because in the HEMT device 1A having the first gate-recess structure, a free surface state 40a (marked with x) formed on the surface of the barrier layer 30 exposed by the recess region R1 exhibits a negatively charged surface state to change the field in a channel 21, thereby reducing sheet carrier density (ns).
Meanwhile, RF characteristics of the HEMT device 1B having the second gate-recess structure have not been improved compared to those of the HEMT device 1A having the first gate-recess structure although the HEMT device 1B has excellent DC characteristics. This is because the cap layer 40 with conductivity is formed up to the vicinity of the gate electrode 53 in the HEMT device 1B having the second gate-recess structure to reduce a substantial distance between the gate electrode and the drain electrode, thereby increasing capacitance (Cgd) between the gate electrode and the drain electrode. Since influence of small-signal parameters on RF characteristics is relatively low compared to that of capacitance (Cgd) between the gate electrode and the drain electrode on RF characteristics, description has focused on capacitance (Cgd).
Thus, there is a need to develop a power semiconductor having excellent DC characteristics and excellent RF characteristics.